As semiconductor devices continue to scale to smaller and smaller dimensions, resistance in the gate electrode lines of field effect transistors becomes a concern. One way to reduce the resistance in a gate electrode is to use a combination of polysilicon and metal. These are known as poly-metal gates. The gate stack consists of a gate dielectric, a polysilicon layer, a barrier layer and a metal layer.
Three oxidation steps are desirable during the gate formation. The first is a chemical vapor deposition (CVD) of silicon dioxide over the metal layer prior to the gate stack etch. The second is a light thermal reoxidation (sometimes referred to as a "poly smile" oxidation) after gate etch to remove the etch damage. The third is a sidewall CVD-SiO2 deposition after the gate etch.
The oxidation of silicon (including polysilicon) is self-limiting in the presence of these three oxidation steps. That is, only a small portion of the silicon will oxidize. Unfortunately, metals, such as tungsten, are not self-limiting. Accordingly, the tungsten may completely oxidize and even vaporize during any of these oxidation processes.
One prior art method of selective oxidation for poly-metal gate formation was proposed by Kobayashi et al (Proc. of 15.sup.th Conf. Solid State Devices and Material p. 217 (1983)). In this method, a wet hydrogen oxidation procedure was developed to allow the silicon to oxidize while leaving the tungsten unaffected in a post gateetch oxidation. The method is based on thermodynamic calculations which show that at, for example, 1000.degree. C. and a P(H2O)/P(H2) ratio (partial pressure ratio of H20 and H2) of 1.0e-05, the equilibrium: EQU Si+2 H2O.rarw..fwdarw.SiO2+2H2
prefers the right side of the reaction, i.e., oxidation of Si and EQU W+3H2O.rarw..fwdarw.WO3+3H3
prefers the left side of the reaction, i.e., reduction of WO3 to W. Therefore, under appropriate conditions, it is possible to oxidize silicon again such that the oxidation rate of W will be prevented.
Unfortunately, it is difficult to generate a uniform steam of pure H2O without heavy metal contaminants because it is hard to completely remove heavy metal from the clean and deionized water used in steam generators for semiconductor device manufacturing. It is also dangerous to generate and control a proper H2/H2O gas ratio using a burning process of mixed oxygen and hydrogen under an excessive hydrogen environment.
The above process has been proposed for light thermal oxidation. Selective CVD of oxide is also desirable in forming poly-metal gates. In the area of non-selective CVD-SiO2, several CO2-H2 gas chemistries have been proposed. One such gas chemistry is SiH4-CO2-H2 and another is SiH2Cl2-CO2-H2. A CO2-H2 gas chemistry has also been used to produce H2O for H2O addition reactions such as 2AlCl3+3H2O.fwdarw.Al2O3+6HCl. In addition, CO gas has been used in metallurgy as a strong reduction reagent of metal oxides for metal production.
There is a need for both selective thermal oxidation and selective CVD oxidation processes that do not significantly oxidize metal.